1. Field of the Invention
The present invention relates generally to a system and a method for a periodic ranging in a Broadband Wireless Access (BWA) communication system, and more particularly to a system and a method for performing a periodic ranging of a Mobile Station (MS) that remains in a sleep mode.
2. Description of the Related Art
In a 4th generation (4G) communication system, which is the next generation communication system, research is being performed to provide users with services having various Qualities of Service (QoS) at a high speed. In particular, in the current 4G communication system, research has been actively pursued to support a high speed service capable of ensuring mobility and QoS in a BWA communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
A representative communication system of the 4G communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and an IEEE 802.16e communication system. The IEEE 802.16a communication system and the IEEE 802.16e communication system utilize an Orthogonal Frequency Division Multiplexing (OFDM) scheme/an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support a broadband transmission network for a physical channel of the wireless MAN system. The IEEE 802.16a communication system considers only a single cell structure and stationary subscriber stations (SSs), which means the system does not accommodate the mobility of the SSs at all. However, the IEEE 802.16e communication system accommodates the mobility of an SS in the IEEE 802.16a communication system. Here, an SS having mobility is referred to as a Mobile Station (MS).
FIG. 1 is a block diagram schematically illustrating a conventional IEEE 802.16e communication system. Referring to FIG. 1, the IEEE 802.16e communication system has a multi-cell structure, i.e., a cell 100 and a cell 150. Further, the IEEE 802.16e communication system includes a Base Station (BS) 110 that controls the cell 100, a BS 140 that controls the cell 150, and a plurality of MSs 111, 113, 130, 151, and 153. The transmission/reception of signals between the BSs 110 and 140 and the MSs 111, 113, 130, 151, and 153 is accomplished using an OFDM/OFDMA scheme.
In FIG. 1, the MS 130 is located in a boundary area (or a handover area) between the cell 100 and the cell 150. That is, when the MS 130 moves into the cell 150 controlled by the BS 140 while communicating with the BS 110, a serving BS of the MS 130 changes from the BS 110 to the BS 140.
Because the IEEE 802.16e communication system accommodates the mobility of an MS, power consumption of the MS is an important factor in the entire system. Accordingly, a sleep mode operation and an awake mode operation corresponding to the sleep mode operation between the MS and the BS have been proposed to minimize the power consumption of the MS. More specifically, the MS periodically performs a ranging operation for compensating for a timing offset, a frequency offset, and power with the BS in order accommodate changes in channel conditions with the BS.
Further, because the IEEE 802.16e communication system accommodates the mobility of an MS, a periodic ranging of the ranging operation is growing more important.
FIG. 2 is a diagram schematically illustrating a conventional sleep mode operation in the IEEE 802.16e communication system. However, before a description of FIG. 2 is given, it should be noted that the sleep mode has been proposed in order to minimize power consumption of an MS in an idle interval, in which packet data are not transmitted, when the packet data are transmitted. That is, in the sleep mode, the MS and a BS simultaneously transit to the sleep mode, thereby minimizing the power consumption of the MS in the idle interval in which the packet data is not transmitted.
More specifically, the packet data is burst when generated. Accordingly, it is unreasonable that the same operation is performed in both an interval in which the packet data are not transmitted and an interval in which the packet data are transmitted. Therefore, the sleep mode operation as described above has been proposed.
When packet data to be transmitted is generated while both the MS and the BS are in the sleep mode, the MS and the BS must simultaneously transit to the awake mode and must transmit/receive the packet data.
The sleep mode operation described above is proposed not only in terms of power consumption but also as a scheme for minimizing interference between channel signals. However, because traffic has a large influence on the packet data character, the sleep mode operation must be performed in consideration of the traffic characteristic, the transmission scheme characteristic, etc., of the packet data.
Referring to FIG. 2, a reference numeral 211 identifies the generation pattern of packet data and includes a plurality of ON intervals and OFF intervals. The ON intervals are burst intervals in which packet data (or traffic) is generated and the OFF intervals are idle intervals in which the traffic is not generated. The MS and the BS are transitted to a sleep mode and an awake mode according to the traffic generation pattern as described above, such that the power consumption of the MS can be minimized and interference between channel signals can be prevented.
Reference numeral 213 identifies the mode transition of a BS and an MS, and includes a plurality of awake modes and sleep modes. In the awake modes, traffic is generated and packet data is exchanged. In the sleep modes, the traffic is not generated and the packet data is not exchanged between the MS and the BS.
Reference numeral 215 identifies the MS power level. As illustrated in FIG. 2, when the MS power level is K in the awake mode, the MS power level is M in the sleep mode. When the MS power level K in the awake mode is compared with the MS power level M in the sleep mode, the M has a value much smaller than that of the K. That is, because the packet data is not exchanged in the sleep mode, the power of the MS is not consumed as much.
In order to transit to the sleep mode, an MS must receive a mode transition approval from a BS. The BS approves a mode transition to a sleep mode of the MS and transmits packet data. Further, the BS must inform the MS that packet data to be transmitted to the MS exists during a listening interval of the MS. Herein, the MS must awake from the sleep mode and confirm if there is packet data to be transmitted from the BS to the MS. The listening interval will be described later in more detail.
As a result of the confirmation by the MS, when there is the packet data to be transmitted from the BS to the MS, the MS transits to the awake mode and receives the packet data from the BS. However, when there is no packet data to be transmitted from the BS to the MS, the MS may return to the sleep mode or maintain the awake mode.
Parameters required for supporting the sleep mode operation and the awake mode operation will be described herein below.
(1) Sleep Identifier (SLPID)
The SLPID proposed by the IEEE 802.16e communication system corresponds to a value allocated through a Sleep-Response (SLP-RSP) message when the MS transits to the sleep mode, which is used as a specific value for only MSs staying in the sleep mode. That is, the SLPID is an identifier for differentiating MSs in the sleep mode including a listening interval. When the corresponding MS transits to the awake mode, an SLPID is restored to the BS and may be reused for an MS intended to transit to the sleep mode through the SLP-RSP message. The SLPID has a size of 10 bits and it is possible to support 1024 MSs performing the sleep mode operation using the SLPID.
(2) Sleep Interval
The sleep interval, which is requested by the MS, may be allocated by the BS according to the request of the MS. The sleep interval is a time interval for which the MS transits to the sleep mode and then maintains the sleep mode until the listening interval starts. The sleep interval may be defined as a time for which the MS stays in the sleep mode.
The MS may continuously stay in the sleep mode when there is no data to be transmitted from the BS to the MS, even after the sleep interval. In such a case, the MS increases and updates the sleep interval by a preset initial-sleep window value and a final-sleep window value. The initial-sleep window value represents an initial minimum value of the sleep interval and the final-sleep window value represents a final maximum value of the sleep interval. The initial-sleep window value and the final-sleep window value may be expressed by the number of frames. The initial-sleep window value and the final-sleep window value will be described later in more detail.
The listening interval, which is requested by the MS, may be allocated by the BS according to the request of the MS. That is, the listening interval is a time interval for which the MS awakes from the sleep mode momentarily, synchronizes with a downlink signal of the BS, and receives downlink messages such as traffic indication (TRF-IND) messages. The TRF-IND message identifies if there is a TRF-IND, i.e., packet data, to be transmitted to the MS. The TRF-IND message will be described later in more detail.
The MS continuously waits to receive the TRF-IND message for the listening interval. If a bit representing the MS in an SLPID bitmap included in the TRF-IND message has a value indicating a positive indication, the MS continuously maintains the awake mode. As a result, the MS transits to the awake mode. However, if the bit has a value indicating a negative indication, the MS transits to the sleep mode again.
3) Sleep Interval Update Algorithm
When the MS shifts to the sleep mode, the MS determines the sleep interval from a preset minimum window value as a minimum sleep mode period. After the MS awakes from the sleep mode for the listening interval and confirms an absence of packet data to be transmitted from the BS, the MS sets the sleep interval to have a value corresponding to twice that of the previous sleep interval, and remains in the sleep mode. For example, when the minimum window value is 2, the MS sets the sleep interval to 2 frames, and remains in the sleep mode for the 2 frames. After the 2 frames pass, the MS awakes from the sleep mode and determines if the TRF-IND message has been received. When the TRF-IND message has not been received, i.e., when there is no packet data transmitted from the BS to the MS, the MS sets the sleep interval to be 4 frames, which is twice as many as 2 frames, and remains in the sleep mode for the 4 frames. Accordingly, the sleep interval may increase from the minimum window value to the maximum window value, and an update algorithm for the sleep interval is the sleep interval update algorithm.
Messages defined in the IEEE 802.16e communication system for supporting the sleep mode operation and the awake mode operation as described above will be described herein below.
(1) Sleep Request (SLP-REQ) Message
The SLP-REQ message is transmitted from an MS to a BS, which is a message used when the MS requests a mode transition to a sleep mode. The SLP-REQ message includes parameters, i.e., information elements (IEs), required when the MS operates in the sleep mode. A format of the SLP-REQ message is shown in Table 1.
TABLE 1SyntaxSizeNotesSLP-REQ_Message_Format ( ) {Management message type = 46 8 bitsinitial-sleep window 6 bitsfinal-sleep window10 bitsListening interval 6 bitsReserved 2 bits}
The SLP-REQ message is a dedicated message transmitted based on a connection ID (CID) of an MS.
The Management message type IE represents the type of message being transmitted. For example, when the Management message type has a value of 45, the transmitted message is the SLP-REQ message.
The initial-sleep window value IE represents a requested start value for the sleep interval (e.g., measured in frames), and the final-sleep window value represents a requested stop value for the sleep interval. That is, as described above for the sleep interval update algorithm, the sleep interval may be updated within a range from the initial-sleep window value to the final-sleep window value.
The listening interval represents a requested listening interval, which may also be expressed by the number of frames.
(2) SLPRSP Message
The SLP-RSP message is a response message for the SLP-REQ message, which can used to indicate whether to approve or deny the mode transition to the sleep mode requested by the MS, or as an unsolicited instruction. The SLP-RSP message includes IEs required when the MS operates in the sleep mode. A format of the SLP-RSP message is shown in Table 2.
TABLE 2SyntaxSizeNotesSLP-RSP_Message_Format ( ) {Management message type = 47 8 bitsSleep-approved 1 bit0: Sleep-mode requestdenied1: Sleep-mode requestapprovedIF (Sleep-approved ==0) { After-REQ-action 1 bit0: The MS may retransmitthe SLP-REQ messageafter time duration(REQduration) given by theBS in this message1: The MS shall notretransmit the -SLP-REQmessage and shall awaitthe -SLP-RSP message fromthe BS REQ-duration 4 bitsTime duration for casewhere After-REQ-action value is 0 Reserved 2 bits } Else {  Start frame  initial-sleep window 6 bits  final-sleep window10 bits  listening interval 6 bits  SLPID10 bits }}
The SLP-RSP message is a dedicated message transmitted based on a basic CID of the MS.
The Management message type IE represents the type of a message currently being transmitted. For example, when the Management message type has a value of 46, the transmitted message represents the SLP-RSP message.
Further, the Sleep-approved has a value expressed by one bit. When the Sleep-approved has a value of 0, it implies that the request for the mode transition to the sleep mode has been denied (SLEEP-MODE REQUEST DENIED). However, when the Sleep-approved has a value of 1, it implies that the request for the mode transition to the sleep mode has been approved (SLEEP-MODE REQUEST APPROVED). Further, when the Sleep-approved has the value of 0, it implies that the BS has denied the mode transition to the sleep mode requested by the MS.
Accordingly, the MS having experienced the denial transmits the SLP-REQ message to the BS or waits for receiving an SLP-RSP message representing an unsolicited instruction from the BS when the situation requires. When the Sleep-approved has the value of 1, there exist the Start frame value, the initial-sleep window value, the final-sleep window value, the listening interval and the aforementioned SLPID. However, when the Sleep-approved has the value of 0, there exist the After-REQ-action value and the REQ-duration.
The Start frame value represents the number of frames, not including the frame in which the SLP-RSP message has been received, until the MS enters the first sleep interval. That is, the MS transits to the sleep mode after the frames corresponding to the start frame value have passed from a frame directly after the frame in which the SLP-RSP message has been received.
The SLPID is used for differentiating MSs staying in the sleep mode, which allows the total 1024 MSs staying in the sleep mode to be distinguished from one another.
As described above, the initial-sleep window value represents a start value for the sleep interval, which is measured in frames, the listening interval represents a value for a listening interval, and the final-sleep window value represents a stop value for the sleep interval. The After-REQ-action value represents an operation, which must be done by the MS having experienced the denial for the mode transition to the sleep mode.
3) TRF-IND Message
The TRF-IND message is a message transmitted from the BS to the MS during the listening interval, which represents the existence of packet data to be transmitted from the BS to the MS. The TRF-IND message has a format as shown in Table 3.
TABLE 3SyntaxSizeNotesTRF-IND_Message_Format ( ) {Management message type = 488 bitsSLPID bit-mapVariable}
The TRF-IND message is a broadcasting message transmitted through the broadcasting scheme, differently from the SLP-REQ message and the SLP-RSP message. The TRF-IND message represents if there is packet data to be transmitted from the BS to a predetermined MS. The MS decodes the broadcasted TRF-IND message during the listening interval and determines whether to transit to an awake mode or to return to the sleep mode again.
When the MS transits to the awake mode, the MS confirms frame sync. When the frame sync does not coincide with a frame sequence number expected by the MS, the MS can request retransmission of packet data lost in the awake mode. When the MS has failed to receive the TRF-IND message during the listening interval or the TRF-IND message having received in the MS does not include a value representing a positive indication, the MS may return to the sleep mode.
The Management message type IE is information representing the type of a message currently being transmitted. For example, when the Management message type has a value of 48, the transmitted message represents the TRF-IND message.
The SLPID bit-map represents a set of indication indices. Each of the indication indices has one bit allocated to one of SLPIDs assigned to MSs in order to identify the MSs, respectively, which have transited to the sleep mode. That is, the SLPID bit-map represents a group of bits, each of which is allocated to an MS in the SLPID values (with a maximum value of ‘−1’) assigned to the MSs currently staying in the sleep mode. The SLPID bit-map may be allocated a dummy bit for a byte alignment.
A bit allocated to the MS represents if there is packet data to be transmitted from the BS to a corresponding MS. Further, the MS in the sleep mode reads an SLPID and a mapped bit in the TRF-IND message received during the listening interval, which have been allocated in the mode transition to the sleep mode. If the allocated bit has a positive indication value, i.e., 1, the MS continuously maintains the awake mode. As a result, the MS transits to the awake mode. However, if the allocated bit has a negative indication value, i.e., 0, the MS transits to the sleep mode again.
FIG. 3 is a flow diagram schematically illustrating a conventional ranging process in the IEEE 802.16e communication system. Referring to FIG. 3, the MS 300 is powered on, monitors all frequency bands having been already set in the MS 300, and detects a reference signal, e.g., a pilot signal, having the highest Carrier-to-Interference and Noise-Ratio (CINR). The MS 300 determines a BS 320 having transmitted the pilot signal having the highest CINR as the BS 320 (or serving BS 320) to which the MS 300 currently belongs. The MS 300 receives the preamble of the downlink frame transmitted from the serving BS 320 and acquires system synchronization with the BS 320.
As described above, when the system synchronization is acquired between the MS 300 and the serving BS 320, the serving BS 320 transmits a DownLink (DL)-MAP message and an Uplink (UL)-MAP message to the MS 300 in steps 311 and 313. The DL-MAP message has a format as shown in Table 4.
TABLE 4SyntaxSizeNotesDL-MAP_Message_Format( ) {Management Message Type=2 8 bitsPHY Synchronization FieldVariableSee Appropriate PHYspecification   DCD Count 8 bits  Base Station ID48 bits Number of DL-MAP Element n16 bitsBegin PHY specific section {See Applicable PHYsection For (i=1; i<=n; i++)For each DL-MAPelement 1 to nDL-MAP Information Element( )VariableSee corresponding PHYspecification  If!(byte boundary) { 4 bitsPadding to reach byte  Padding Nibbleboundary    }   }  } }
As shown in Table 4, the DL-MAP message includes a plurality of IEs, that is, the Management Message Type representing the type of a transmitted message, the PHYsical (PHY) Synchronization set according to a modulation scheme and a demodulation scheme applied to a physical channel in order to acquire synchronization, the DCD count representing a count corresponding to the configuration variation of a Downlink Channel Descriptor (DCD) message including a downlink bust profile, the Base Station ID representing a Base Station identifier, and the ‘Number of DL-MAP Elements n’ representing the number of elements existing after the Base Station ID. In particular, the DL-MAP message includes information for ranging codes allocated to each ranging in an OFDMA communication system. The MS 300 may detect information for downlink bursts included in the downlink frame through the DL-MAP message. Accordingly, the MS 300 may receive data, that is, data frames, in the burst by differentiating the downlink bursts of the downlink frame.
The UL-MAP message has a format as shown in Table 5.
TABLE 5SyntaxSizeNotesUL-MAP_Message_Format( ) {Management Message Type=3 8 bits  Uplink Channel ID 8 bits   UCD Count 8 bitsNumber of UL-MAP Element n16 bits  Allocation Start Time32 bits Begin PHY specific section {See ApplicablePHY section  for (i=1; i<=n; i++)For each UL-MAPelement 1 to nUL-MAP_Information_Element( )VariableSee correspondingPHY specification       }      }    }
As shown in Table 5, the UL-MAP message includes a plurality of IEs, that is, the Management Message Type representing the type of a transmitted message, the Uplink Channel ID representing a used uplink channel identifier, the UCD count representing a count corresponding to the configuration variation of an Uplink Channel Descriptor (UCD) message including an uplink bust profile, and the ‘Number of UL-MAP Elements n’ representing the number of elements existing after the UCD count. The uplink channel identifier is uniquely allocated by a Medium Access Control (MAC) sub-layer.
The MS 300 having synchronized with the BS 320, i.e., the MS 300 having recognized downlink and uplink control information and actual data transmission/reception locations, transmits a Ranging Request (RNG-REQ) message to the BS 320 in step 315. The BS 320 having received the RNG-REQ message transmits a Ranging Response (RNG-RSP) message, which includes information for compensating for a frequency, a time and transmit power for the ranging, to the MS 300 in step 317.
In FIG. 3, for convenience of description, the ranging process is ended through one-time RNG-REQ message transmission process and one-time RNG-RSP message transmission process corresponding to the RNG-REQ message transmission. However, according to the actual ranging process, the RNG-REQ message transmission process and the RNG-RSP message transmission process corresponding to the RNG-REQ message transmission may be repeated several times until the transmit power/timing/frequency compensation for the uplink is completed. The ranging process is periodically performed.
The RNG-REQ message has a format as shown in Table 6.
TABLE 6SyntaxSizeNotesRNG-REQ_message_Format( ) { Management Message Type=48 bits Downlink Channel ID8 bits Pending Until Complete8 bits TLV Encoded InformationVariableTLV specific}
As shown in Table 6, the RNG-REQ message includes a plurality of IEs, that is, the Management Message Type representing the type of a transmitted message, the Downlink Channel ID representing a downlink channel identifier included in the RNG-REQ message received in the MS 300 through the UCD message, and the Pending Until Complete representing a priority of a transmitted ranging response. The Pending Until Complete has a value of 8 bits. When the Pending Until Complete has a value of ‘00000000’, the previous ranging response has a high priority. However, when the Pending Until Complete does not have the value of ‘00000000’, the current ranging response has a high priority.
The RNG-RSP message has a format as shown in Table 7.
TABLE 7SyntaxSizeNotesRNG-RSP_message_Format( ) { Management Message Type=58 bits Uplink Channel ID8 bits TLV Encoded InformationVariableTLV specific}
As shown in Table 7, the RNG-RSP message includes a plurality of IEs, that is, the Management Message Type representing the type of a transmitted message, and the Uplink Channel ID representing an uplink channel identifier included in the RNG-REQ message.
The completion of the transmission/reception operations of the RNG-REQ message and the RNG-RSP message, i.e., the completion of the ranging process, may be determined by a Ranging Status parameter value of the TLV (Type, Length, and Value) Encoded Information as shown in Table 7. The Ranging Status parameter has one of the values as shown in Table 8.
TABLE 8Value of Ranging StatusMeaning1Continue2Abort3Success4Re-Range
The ranging process is performed through at least one-time exchange of the RNG-REQ message and the RNG-RSP message as described above. More specifically, the exchange of the RNG-REQ message and the RNG-RSP message may be repeated until the transmit power/timing/frequency compensation is completed. Further, the exchange of the RNG-REQ message and the RNG-RSP message of more than twice is controlled by the value of the Ranging Status in the RNG-RSP message transmitted from the BS.
When the Ranging Status in the RNG-RSP message transmitted from the BS has a value of 1, the MS determines that it is necessary to additionally exchange the RNG-REQ message and the RNG-RSP message. More specifically, the MS determines that the ranging process continues, performs the transmit power/timing/frequency compensation with the BS, and then transmits the RNG-REQ message to the BS. The BS having received the RNG-REQ message from the MS sets the Ranging Status of the RNG-RSP message to have a value of 1 again when an additional compensation is required according to status of the transmit power/timing/frequency compensation by the MS. The BS transmits the RNG-RSP message to the MS and enables an additional exchange the RNG-REQ message and the RNG-RSP message to be performed.
However, when the additional compensation is not required according to the status of the transmit power/timing/frequency compensation by the MS, i.e., the ranging process has succeeded, the BS sets the Ranging Status of the RNG-RSP message to have a value of 3 and prevents the RNG-REQ message and the RNG-RSP message from being additionally exchanged.
Hereinafter, the ranging will be described in detail.
The ranging may be classified into an initial ranging, a maintenance ranging, i.e., a periodic ranging, and a bandwidth request ranging. The MS may compensate for the transmit power through the ranging operation before transmitting data through an uplink, and may compensate for the timing offset and the frequency offset.
First, the initial ranging will be described.
The initial ranging is performed when a BS acquires synchronization with an MS, which represents a ranging performed in order to match the exact time offset between the MS and the BS and compensate for the transmit power. That is, the MS is powered on, receives a DL-MAP message and an UL-MAP message, and acquires synchronization with the BS. The MS performs the initial ranging to compensate for the time offset and the transmit power with the BS.
Second, the periodic ranging will be described.
The periodic ranging is performed when the MS having compensated for the time offset and the transmit power with the BS through the initial ranging compensates for channel conditions, etc., with the BS.
Third, the bandwidth request ranging will be described.
The bandwidth request ranging is performed when the MS having compensated for the time offset and the transmit power with the BS through the initial ranging requests a bandwidth allocation in order to actually perform communication with the BS.
As described above, because the IEEE 802.16e communication system accommodates the mobility of the MS, the periodic ranging of the MS becomes a vital factor for data transmission/reception. According to the periodic ranging, which is an operation for measurement and compensation of parameters required when the MS performs reliable communication with the BS, the BS must allocate uplink resources so that the MS can perform the periodic ranging, i.e., the MS can transmit an RNG-REQ message to the BS. More specifically, the BS must allocate the uplink resources to the MS for the periodic ranging of the MS and notifies information for allocation of the uplink resources of the MS through the UL-MAP message.
Thereafter, the MS transmits the RNG-REQ message to the BS through the allocated uplink resources and performs the periodic ranging operation with the BS. The BS compensates for the transmit power, timing offset, and frequency offset according to the RNG-REQ message received from the MS and transmits the RNG-RSP message to the MS in response to the RNG-REQ message, thereby ending the periodic ranging.
However, because the sleep mode operation and the ranging operation, particularly, the periodic ranging operation, have been proposed to operate independently from each other in the IEEE 802.16e communication system, the sleep mode operation and the periodic ranging operation do not have a correlation between themselves. That is, even an MS staying in the sleep mode must perform the periodic ranging in order to perform reliable communication with the BS. However, because the MS staying in the sleep mode cannot receive a message transmitted from the BS, it is impossible to receive resources for the periodic ranging. Accordingly, it is necessary to propose a scheme for the periodic ranging of the MS staying in the sleep mode.